DC/DC conversion circuit

Abstract: Of the type that implements the "forward PWM" topology and which comprises a DC/AC converter (M), a self-driven synchronous rectifier (T1,T2), which rectifies the alternating voltage signal coming from the DC/AC converter (M), and a filter (L1,C1) that filters the rectified signal, thereby obtaining the DC output voltage from the DC/DC conversion circuit. This DC/DC conversion circuit is characterised in that the DC/AC converter (M) includes means to generate an alternating signal with a square waveform that does not take zero instantaneous values such that no current flows through the body diode of either of the transistors (T1,T2) of the self-driven synchronous rectifier.

Description: OBJECT OF THE INVENTION This invention, according to the title of this document, refers to a highly efficient DC/DC conversion circuit with "forward PWM" topology which comprises a DC/AC converter, a self-driven synchronous rectifier and a filter, and which increases the power efficiency of the whole assembly. The circuit, according to the invention, is of particular application to direct current supply converters for low output voltages where the efficiency is greatly affected by losses in the rectification stage. BACKGROUND TO THE INVENTION There are several topologies used for implementation of DC/DC converters. An example is given in PCT WO 83/02858 . Other DC/DC converters comprise a DC/AC converter, a rectifier stage with two conventional diodes and a filter. For example, US-A-4 809 148 describes such a DC/DC converter. This DC/DC converter does not present a high overall efficiency because there are voltage drop in the diodes of the rectifier stage, being important for low output voltage applications below 5V. The conduction losses of the rectifier stage can be reduced and hence, overall DC/DC converter efficiency increased by using MOSFETs like synchronous rectifiers. A DC/DC conversion circuits is disclosed in the relevant prior art " Practical application of MOSFET synchronous rectifiers" by James Blanc in INTELEC'91, Nov. 5-8, 1991, Kyoto, Jp . This DC/DC conversion circuit comprises a DC/AC converter, a self-driven synchronous rectifier with two MOSFETs and a filter. The DC/AC converter is of the forward type with resonant reset circuit. Unfortunately, this self-driven synchronous rectifier has limitations in the way in which the MOSFETs can be driven due to the conduction of the body diode. TECHNICAL PROBLEM TO BE OVERCOME Consequently the technical problem to be overcome consists in preventing the flow of current through the body diode of the MOSFET transistors in the self-driven synchronous rectifier, thereby reducing its losses and enhancing the overall efficiency of the whole assembly. CHARACTERISATION OF THE INVENTION The DC/DC conversion circuit, in accordance with the invention, comprise a DC/AC converter that includes means for generating an alternating signal with a square waveform that does not take zero instantaneous values, a self-driven synchronous rectifier and a filter. In this way, the signal coming from the transformer included in the DC/AC converter makes that the transistors of the self-driven synchronous rectifier are not simultaneously in the cut-off state, which would result in current flowing through the body diode of one of the field-effect transistors. As a consequence, the losses that would arise in this component are considerably lessened and the overall efficiency of the DC/DC converter increased, this increase in practical circuits reaching approximately 4 points. BRIEF FOOTNOTES TO THE FIGURES A more detailed explanation of the invention is given based on the following figures in which: figure 1 shows a DC/DC conversion circuit consisting of a DC/AC converter, a self-driven synchronous rectifier and a filter, figures 2.1 and 2.2 show the waveforms at the input terminals of the self-driven synchronous rectifier in accordance with the state of the art and with the invention respectively, figures 3.1 and 3.2 show two possible implementations of the DC/AC converter that makes use of an active limiter in order to demagnetise the transformer, all in accordance with the invention, figures 4.1 and 4.2 show two other possible implementations of the DC/AC converter that make use of a passive limiter, and figure 5 shows an alternative implementation of the DC/AC converter that includes a demagnetising circuit for the transformer using an auxiliary winding. DESCRIPTION OF THE INVENTION The DC/DC conversion circuit of figure 1 consists of a DC/AC converter, M, that generates an alternating signal across its output terminals I1 and I2, that has to be rectified by a rectifier circuit and filtered by a filter in order to obtain the required DC voltage at its output O 1 ,O 2 . In this case, use is made of a self-driven synchronous rectifier T1,T2 and a filter L1,C1 that considerably reduces the losses in the rectifier elements (transistors T1 and T2 instead of diodes). However, so that the reduction can be as great as possible, it is necessary to avoid the problem stated above whereby the body diode of transistor T2 conducts during the period of time when the voltage across the input terminals I1 and I2 of the self-driven synchronous rectifier is zero. To avoid the situation described above, the DC/AC converter, M, does not make use of a conventional demagnetising procedure for the output transformer, which would result in an alternating signal with a waveform of the type shown in figure 2.1, instead it includes means to generate an alternating signal with a square waveform that does not take zero instantaneous values, of the kind shown in figure 2.2, which avoids a situation of simultaneous cutt-off of both transistors T1,T2. Figure 3.1 shows a possible implementation of the DC/AC converter, M, that consists of a transformer TR1 which has one of its windings connected between a DC supply voltage Vi and the drain of a transistor T3 which, in turn, has its source connected to the reference voltage V0, in such a way that this transistor T3 is brought to the cut-off or switched-on state by the application of a control voltage Vc1, with a square waveform, coming from a control circuit CC1. The pulsed current flowing in this winding acts to magnetise the transformer TR1, which has to be demagnetised in some way to prevent its saturation . This is done making the current flowing through it to go down or, what is the same thing, the derivative of the current becoming negative, such that the mean value of this current does not grow indefinitely, but remains at a determined level around which the instantaneous current varies. According to the invention, this is achieved by means of a magnetising return current module MRC which is an active limiter formed by the capacitor C2 in series with the drain-source junction of the transistor T4, the latter operating in the cut-off or switched-on state through the application to its gate of a control signal Vc2 coming from the control circuit CC1, this being a complementary signal to the control signal Vc1. This magnetising return current module MRC is connected between the node that joins the drain of the transistor T3 to one of the terminals of one of the windings of TR1 and a constant voltage node which, in this case, is the DC voltage supply Vi. When T3 is conducting, the transformer TR1 is magnetised, but while T4 is conducting the transformer is demagnetised. Figure 3.2 is a variation on figure 3.1, in which the constant voltage node to which the magnetising return current module MRC is connected, is, in this case, the reference voltage V0, this module consisting of the capacitor C3 and the transistor T5, its operation being entirely analogous to the previous case. Figure 4.1 shows another possible implementation of the DC/AC converter, M, in which the magnetising return current module MRC is a passive limiter formed by the resistor R1 in parallel with the capacitor C4 and this assembly in series with the diode D1. The cathode of this diode D1 is connected to the node N1 that joins one of the terminals of C4 to one of the terminals of R1 and its anode is connected to the node that joins the drain of T3 to one of the terminals of a winding of TR1; the other terminal of the magnetising return current module MRC, which coincides with the other terminals of C4 and Cl, is connected to a constant voltage node which, in this case, is the input voltage Vi. Similarly, in figure 4.2, the equivalent circuit is shown for the case that the constant voltage node to which one of the terminals of the magnetising return current module MRC is connected, is the reference voltage V0, formed in this case by R2, C5 and D2. The previous circuits have been implemented using N-channel MOSFET transistors though it must be clear to any expert in the matter that such circuits need not be limited to this type of transistor and can be replaced by others with the corresponding adaptation to the types of bias required (for example, P-channel MOSFETs).

Claims: DC/DC forward conversion circuit comprising a DC/AC converter (M) including a transformer (TR 1 ) comprising a primary winding having a primary winding first end and a primary winding second end, the primary winding second end being connected to a DC supply voltage (Vi), and a secondary winding having a secondary winding first end (I 1 ) and a secondary winding second end (I 2 ), a main MOSFET transistor (T 3 ) having a drain connected to the primary winding first end, a source connected to a reference voltage (V 0 ), a self-driven synchronous rectifier comprising a first rectifier output (O 2 ) and a second rectifier output (O 1 ), the second rectifier output being connected to the secondary winding second end (I 2 ), a rectifier first MOSFET transistor (T 1 ) connected between the secondary winding first end (I 1 ) and the second rectifier output (O 1 ) and having a first gate connected to the secondary winding second end (I 2 ), a rectifier second MOFSET transistor (T 2 ) connected between the fire rectifier output (O 2 ) and the second rectifier output (O 1 ), and having a second gate connected to the first end of the secondary winding (I 1 ) a LC filter (L 1 ,C 1 ) connected to the first and second rectifier outputs, characterized in that said DC/AC converter (M) comprises a magnetizing return current module (MRC), so that it generates, at said secondary winding first and second ends (I 1 , I 2 ), a square alternating control signal V sec , which does not take zero instantaneous values for driving the gates of said rectifier first and second MOSFET transistors (T 1 , T 2 ), preventing said rectifier first and second MOSFET transistors (T 1 ,T 2 ) to be simultaneously cut-off. DC/DC conversion circuit according to claim 1 characterized in that said magnetizing return current module (MRC) comprises a capacitor (C 2 ) in series with a magnetizing return current MOFSET transistor (T 4 ) connected in parallel with the primary winding. DC/DC conversion circuit according to claim 1 characterized in that magnetizing return current module (MRC) comprises a capacitor (C 3 ) in series with a magnetizing return current MOFSET transistor (T 5 ) connected in parallel with the main MOSFET transistor (T 3 ).

Inventors: DE LA CRUZ MORENO ENRIQUE, RODRIGUEZ PEREZ JULIO, VAZQUEZ LOPEZ MANUEL, DE LA CRUZ MORENO, ENRIQUE, RODRIGUEZ PEREZ, JULIO, VAZQUEZ LOPEZ, MANUEL
Applicants: CIT ALCATEL, ALCATEL
Application year: 1994
Main classification: H02M3
IPC codes: H02M3, H02J3, H02M7